This invention relates to elevated concrete flat plate slab constructions, and more particularly to post-tensioned concrete concrete flat plate slab constructions which include a plurality of embedded, laterally spaced apart, tensioned uniform cable tendons extending in a first, longitudinal direction across the slab and a plurality of longitudinally spaced apart, tensioned banded cable tendons extending in a lateral direction over each row of support columns supporting the slab in elevated condition. In particular, the present invention provides a tendon-identifying post tensioned slab construction that effectively utilizes and combines the structural aspects of one-way slab type construction and two-way slab construction. These slab floor constructions are well known and utilized in the construction of elevated building floors, bridges, multi-level parking structures and many other such structures.
As is well understood in the industry, one type of two-way slab construction is a post-tensioned flat plate slab system which typically provides a concrete slab having a generally uniform thickness throughout. Other generally similar two-way labs may have increased thicknesses at their juncture with and extending between the underlying floor support columns which support them in elevated condition, as s well known.
In post-tensioned flat plate slab construction, preliminary to pouring the concrete for the slab on the temporary concrete formwork, a plurality of individual or bundled post-tensioning cables are positioned for extension from one peripheral edge of the formwork to the opposite longitudinal edge. These cables, known as uniform tendons, are placed at regular laterally-spaced apart points, typically at 36 inches, across the formwork. A plurality of bundled cables, known as banded tendons, are positioned for extension in the direction perpendicular to the extension of the uniform tendons, and placed at longitudinally spaced apart positions along the formwork overlying and extending along each row of underlying floor support columns as is known.
Once the concrete slab has been poured and has hardened, the uniform and banded tendons are placed under a selected pull tension by a hydraulic ram and secured in tension condition, thereby creating the post-tensioned flat plate slab construction. The applied stressing force of the tensioned tendons imparts an overall compressive stress on the slab, and the generally undulating extension of the tendons provide for upward or downward reacting forces depending on the particular configuration. In this regard the post-tensioned tendons create the necessary reacting strength to resist gravity-imposed loading on building structures, typically for floors and roofs.
As previously mentioned, the flat plate slab construction is a two-way type slab construction in which the unsupported span portions are effectively supported so that bending stress in the center of the span area is substantially biaxial. In one-way slab construction, the span between supports extends substantially in one direction, and the bending stress in the center of the span is substantially uniaxial. There is intrinsic structural advantage with two-way action since a uniform thickness homogeneous slab has more stiffness and strength with effective support on four sides of the span than one of the same size with supports on only two.
Further, building codes also distinguish between one-way and two-way tensioned concrete slabs, applying more stringent requirements on one way slabs because of their lesser redundancy and robustness. One-way type slab constructions typically utilize a thinner slab thickness supported by underlying joist members, which may comprise thickened portions of the concrete, extending in one direction at pre-engineered laterally spaced points determined by the architect according to engineering load factors. These supporting joists have no relation to the tendon layout or placement. A two-way flat plate slab type construction provides, as the name suggestions, a uniform thickness flat slab, substantially smooth on top and bottom side surfaces, supported on the underlying columns, the slab generally having a greater uniform thickness than the one-way type slab to provide the necessary rigidity required in the spans between support columns.
In both one-way and two-way post-tensioned slab constructions, the uniform and banded tendons are embedded within the interior of the concrete slab in their own predetermined layout and arrangement which is obviously completely hidden from view from either above or below the finished elevated concrete slab construction. However with it understood that, both during construction of a building and afterwards in renovation, etc., workers often need to drill into the bottom side of a floor slab for securing attachments and for drilling through the floor for electrical, plumbing, air handling and other reasons. It is therefore easy for one to understand how necessary it is to assure that a person does not drill or bore into the slab and inadvertently strike or sever one of the highly-tensioned post tensioning tendons in the process. Heretofore in order to identify the location of tensioning tendons to determine safe and suitable places to drill and bore, it has been necessary to enlist the use of X-ray or ground penetrating radar devices. Other attempts have been made to identify the location of post tensioning cables by the provision of dedicated, permanently embedded marker devices installed during the placement of the uniform and banded tendons. These devices provide visual marking of the extending tendons by providing a projecting member which extends beneath the bottom surface of the concrete slab, such as that disclosed in U.S. Pat. No. 4,979,462 to Kramer et al.